We propose to evaluate the way in which cell surface IgM is anchored to the membrane of human B lymphocytes and to examine the possibility that it is associated with other molecular components or fragments within the membrane. The objectives of this study are the following: 1. To isolate cell surface IgM from the membranes of human B lymphocytes; 2. To characterize the molecule in terms of its primary structure, as related to secreted IgM; 3. To evaluate the disulfide linkages, particularly as they relate to the Cys 414 and 565 residues; 4. To evaluate the possibility that additional covalently-linked peptides may be associated with surface IgM; 5. To evaluate the presence of J-chain, Ia antigens or other molecules that might be disulfide bonded to the cell surface IgM; 6. To evaluate the potential of covalent cross-linking of IgM with other surface molecules in order to gain insight into the structural topography of IgM display on the cell surface; 7. To structurally compare in the same cell line secreted IgM with surface IgM.